In electronics, a latch is a kind of bistable multivibrator, an electronic circuit which has two stable states and thereby can store one bit of information. Today the word is mainly used for simple transparent storage elements, while slightly more advanced non-transparent (or clocked) devices are described as flip-flops. Informally, as this distinction is quite new, the two words are sometimes used interchangeably.
When using static gates as building blocks, the most fundamental latch is the simple SR latch (or simple SR flip-flop), where S and R stand for set and reset. Other types include D, JK, T and Clocked Flip-Flops. It can be constructed from a pair of cross-coupled NAND (negative AND) and NOR (negative OR) type logic gates. The stored bit is present on the output marked Q. Normally, in storage mode, the S and R inputs are both low, and feedback maintains the Q and !Q outputs in a constant state, with !Q meaning the complement of Q. If S (Set) is pulsed high while R is held low, then the Q output is forced high, and stays high when S returns low; similarly, if R (Reset) is pulsed high while S is held low, then the Q output is forced low, and stays low when R returns low.
FIG. 1 shows an exemplary synchronous SR latch 10. The synchronous SR latch 10 (sometimes clocked SR flip-flop) can be made by adding a second level of NAND gates to the inverted SR latch (or a second level of NOR gates to the direct SR latch). The extra gates further invert the inputs so the simple SR latch becomes a gated SR latch (and a simple SR latch would transform into a gated SR latch with inverted enable). With E high (enable true) and with the correct clock C transition edge, the signals can pass through the input gates to the encapsulated latch. With E low (enable false) the latch is closed and remains in the state it was left the last time E was high and a rising edge of clock C, for example.
A circuit incorporating latches has state; its output may depend not only on its current input, but also on its previous inputs. Such a circuit is described as sequential logic. A plurality of latches is coupled to one or more global control signals to set, reset and clock storage data.